Lithium secondary battery comprising electrolyte

ABSTRACT

The present invention relates to a lithium secondary battery. The lithium secondary battery includes a positive electrode including a positive active material; a negative electrode including a negative active material; and an electrolyte including a non-aqueous organic solvent, a lithium salt, and an additive including a compound represented by Chemical Formula 1. The negative active material includes Si at about 0.1 wt % to about 32 wt % in amount based on a total weight of the negative active material. 
     
       
         
         
             
             
         
       
         
         
           
             wherein, in Chemical Formula 1, A is a substituted or unsubstituted aliphatic chain or (—C 2 H 4 —O—C 2 H 4 -)n, and n is an integer from 1 to 10.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. patent application Ser. No. 16/335,240, filed on Mar. 20, 2019, which is a National Phase Patent Application of International Patent Application Number PCT/KR2017/009934, filed on Sep. 11, 2017, which claims priority of Korean Patent Application No. 10-2016-0126806, filed on Sep. 30, 2016. The entire contents of all of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a lithium secondary battery.

BACKGROUND ART

A portable information device such as a cell phone, a laptop, a smart phone, and the like or an electric vehicle has used a lithium secondary battery having high energy density and easy portability as a driving power source.

In general, a lithium secondary battery is manufactured by using materials capable of reversibly intercalating and deintercalating lithium ions as a positive active material and a negative active material and filling an electrolyte between the positive electrode and the negative electrode.

Lithium-transition metal oxides are used as the positive active material of the lithium secondary battery, various types of carbon-based materials are used as the negative active material, and lithium salts dissolved in the non-aqueous organic solvent are used as an electrolyte.

In particular, as a lithium secondary battery exhibits battery characteristics by complex reactions such as those between a positive electrode and an electrolyte, a negative electrode and an electrolyte, and the like, the use of a suitable electrolyte is one of important parameters for improving the performance of a lithium secondary battery.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

Embodiments provide a lithium secondary battery exhibiting excellent storage safety while maintaining good cycle-life characteristics.

Technical Solution

In one aspect, the present disclosure provides a lithium secondary battery including: a positive electrode including a positive active material; a negative electrode including a negative active material; and an electrolyte including a non-aqueous organic solvent, a lithium salt, and an additive including a compound represented by Chemical Formula 1, wherein the negative active material includes Si at about 0.1 wt % to about 32 wt % in amount based on a total weight (i.e., 100 wt %) of the negative active material.

In Chemical Formula 1, A is a substituted or unsubstituted aliphatic chain or (—C₂H₄—O—C₂H₄-)n, and n is an integer from 1 to 10.

Advantageous Effects

According to the embodiments, while cycle-life characteristics are maintained, the storage safety of a lithium secondary battery may be improved.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lithium secondary battery according to an embodiment of the present disclosure.

FIG. 2 shows CV characteristic evaluation results of Example 1.

FIG. 3 shows CV characteristic evaluation results of Comparative Example 1.

FIG. 4A shows LSV evaluation results of Example 1 and Comparative Example 1.

FIG. 4B is an enlarged view showing a y-axis scale of FIG. 4A.

FIG. 5 shows the results of measurement of cycle characteristics for Example 2 and Comparative Example 2.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. However, this disclosure may be embodied in many different forms and is not construed as limited to the example embodiments set forth herein.

In order to clearly illustrate the present invention, parts that are not related to the description are omitted, and the same or similar components are denoted by the same reference numerals throughout the specification.

Sizes and thicknesses of components in the drawings are arbitrarily expressed for convenience of description and, thus, the present invention is not limited by the drawings.

In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements.

An electrolyte of a lithium secondary battery may be generally an organic solvent in which a lithium salt is dissolved. In particular, non-aqueous organic solvents having a high ionic conductivity and dielectric constant and a low viscosity may be used.

In lithium secondary battery using a carbonate-based solvent of such a non-aqueous organic solvent, an irreversible side reaction may occur between the electrolyte and the positive electrode, and between the electrolyte and the negative electrode at a high temperature and a high pressure.

The decomposition products produced by such a side reaction form a thick passivation film which acts as a resistance on the surface of the electrode and decrease a cycle life and capacity of the lithium secondary battery. Also, due to the decomposition of a carbonate-based organic solvent, gas is generated inside the battery which causes a swelling phenomenon and it may lead to battery explosion.

A lithium secondary battery according to one embodiment includes, as an electrolyte for a lithium secondary battery for preventing or reducing the swelling phenomenon of a lithium secondary battery and having excellent stability and cycle-life characteristics, an electrolyte including a compound represented by Chemical Formula 1 as an additive.

That is, the lithium secondary battery according to one embodiment includes a positive electrode including a positive active material, a negative electrode including a negative active material, and an electrolyte including a non-aqueous organic solvent, a lithium salt, and an additive including a compound represented by Chemical Formula 1. Furthermore, the negative active material includes Si at about 0.1 wt % to about 32 wt % in amount based on the total weight of the negative active material (i.e., based on 100 weight percent of the negative active material).

In Chemical Formula 1, A is a substituted or unsubstituted aliphatic chain or (—C₂H₄—O—C₂H₄-)n, and n is an integer from 1 to 10.

In Chemical Formula 1, A may be a C₂ to C₂₀ hydrocarbon chain or (—C₂H₄—O—C₂H₄-)n, and n may be an integer from 1 to 5.

In addition, the compound represented by Chemical Formula 1 may be a compound represented by Chemical Formula 1-1.

The negative active material includes Si at an amount of about 0.1 wt % to about 32 wt % based on 100 wt % of the total weight of the negative active material, and such a negative active material may be: i) a material including a core including a mixture of crystalline carbon and Si, and an amorphous carbon coating layer surrounding the core; or ii) a material including a core including crystalline carbon and Si adhered to a surface of the core.

As described above, the lithium secondary battery according to one embodiment includes a negative electrode including a negative active material with Si at an amount of about 0.1 wt % to about 32 wt % based on 100 wt % of the total weight of the negative active material, and an additive including a compound represented by Chemical Formula 1. When the electrolyte including the additive including the compound represented by Chemical Formula 1 is applied to the lithium secondary battery including the negative active material with Si at the range above (e.g., at the specific amount), the cycle-life characteristics of the lithium secondary battery may be improved and the effect for reducing the generation of gas at a high temperature may be enhanced or maximized and the resistance increase (e.g., increase in resistance value) during high-temperature storage may be greatly reduced, while the improvements in the cycle-life characteristics of the lithium secondary battery may be maintained. This is because the Si-containing (i.e., Si-included) negative active material increases the capacity of the battery, but the amount of the gas generation during high-temperature storage may be large and the resistance increase during high-temperature storage may also be large. However, the effects derived from the additive of Chemical Formula 1, including reducing the amount of gas generated at a high temperature and reducing the resistance increase during high-temperature storage, may be obtained (e.g., largely obtained). Thus, the use of the electrolyte with the additive of Chemical Formula 1 allows the lithium secondary battery to exhibit high capacity owing to the Si-included negative active material and to have the improved storage safety, especially, to have the improved high-temperature storage safety and the reduced high storage resistance increase. That is, the use of the electrolyte with the additive of Chemical Formula 1 in combination with the Si-containing negative active material allows the lithium secondary battery to exhibit both high capacity (due to the Si-containing negative active material) and the improved storage safety, especially, the improved high-temperature storage safety, and the reduced high storage resistance increase. Furthermore, the effects related to the improvement during high-temperature storage safety and the decrease in the high-temperature resistance increase may be obtained in the lithium secondary battery including a negative active material with Si at an amount of about 0.1 wt % to about 32 wt %. Even if the electrolyte including the compound of Chemical Formula 1 as the additive is used, if a negative active material with Si at an amount of more than about 32 wt % is used, the amount of gas generation in high-temperature storage is large (e.g., enlarged) and the resistance increase is high so that it is not desirable. In addition, the effects for improving the high-temperature storage safety and reducing the high-temperature resistance increase may be enhanced (e.g., at a maximum) for the lithium secondary battery including a negative active material with Si at an amount of about 0.1 wt % to about 32 wt % and an electrolyte with an additive of Chemical Formula 1 at an amount of about 0.1 wt % to about 3 wt %.

Without being bound to any particular theory, the effects in which the compound of Chemical Formula 1 reduces the amount of the gas generation may be that the compound represented by Chemical Formula 1 includes a difluorophosphite (—OPF₂) group having excellent electrochemical reactivity at both terminals. That is, the compound represented by Chemical Formula 1 includes the difluorophosphite (—OPF₂) group with excellent electrochemical reactivity at both terminals, which leads to the reduced amount of gas generation.

During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide, i.e., the positive active material, are transported into a negative electrode and intercalated thereinto. Because of its high reactivity, lithium reacts with the negative electrode to produce Li₂CO₃, Li₂O, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This thin film is referred to as a solid electrolyte interface (SEI) film. The SEI thin film formed during the initial charge prevents the reaction between lithium ions and the negative electrode or other materials during charge and discharge. In addition, it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the negative electrode, which is caused by co-intercalation of organic solvents of the electrolyte, having a high molecular weight along with solvated lithium ions into the negative electrode. Once the SEI thin film is formed, lithium ions do not react again with the negative electrode or other materials, such that the amount of lithium ions is reversibly maintained. Therefore, in order to improve the high-temperature cycle characteristics and the low-temperature output of the lithium secondary battery, a rigid SEI thin film must be always formed on the negative electrode of the lithium secondary battery.

However, when the additive including the compound represented by Chemical Formula 1 is included like (e.g., as in) the electrolyte for a lithium secondary battery according to the present disclosure, a rigid SEI film having good ion conductivity is formed on the surface of the negative electrode, and thereby it is possible to suppress a decomposition of the surface of the negative electrode during high temperature cycle operation and to prevent an oxidation reaction of the electrolyte solution.

When the compound represented by Chemical Formula 1 is decomposed, a difluorophosphite (—OPF₂) group and an ethylene dioxide fragment may be formed.

The difluorophosphite (—OPF₂) group may form a donor-acceptor bond with transition metal oxide that is exposed on the surface of the positive active material due to excellent electrochemical reactivity and thus a protective layer in a form of a composite may be formed.

In addition, the difluorophosphite (—OPF₂) adhered to the transition metal oxide at the initial charge of the lithium secondary battery may be oxidized to a plurality of fluorophosphates, and thus a more stable inactive layer having excellent ion conductivity may be formed on the positive electrode. Therefore, it is possible to prevent other components of the electrolyte from being oxidation-decomposed, and as a result, the cycle-life performance of the lithium secondary battery may be improved and a swelling phenomenon may be prevented from occurring.

Further, the compound represented by Chemical Formula 1 and its oxide participate in the electrochemical reaction with the components of the SEI thin film to make the SEI thin film more rigid and to improve stability of other components included in the electrolyte by oxidative decomposition.

In addition, the compound represented by Chemical Formula 1 forms a composite with LiPF₆ and thus undesirable side reactions may be prevented from occurring, and it is possible to improve cycle-life characteristics of the lithium secondary battery and to prevent the generation of gas in the lithium secondary battery, thereby significantly reducing an occurrence rate of defects due to a swelling phenomenon.

On the other hand, the additive including the compound represented by Chemical Formula 1 may be included in an amount of 0.1 wt % to 3 wt % based on a total amount of the electrolyte for a lithium secondary battery. More specifically, the amount of the compound represented by Chemical Formula 1 may be 0.1 wt % to 2 wt % or 0.15 wt % to 2 wt %. When the amount of the additive satisfies these ranges, a resistance increase may be prevented, and thus a lithium secondary battery having improved cycle-life characteristics may be realized.

The additive for a lithium secondary battery of this disclosure may further include an additional additive. The additional additive may be, for example, at least one selected from the group consisting of fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, succinonitrile, hexane tricyanide, lithium tetrafluoroborate, and propane sultone, but is not limited thereto.

Herein, an amount of the additional additive may be 0.1 wt % to 20 wt % based on a total amount of the electrolyte for a lithium secondary battery. More specifically, the amount of the additional additive may be 0.1 wt % to 15 wt %. When the amount of the additional additive satisfies these ranges, battery resistance may be effectively suppressed and a lithium secondary battery having suitable cycle-life characteristics may be realized.

On the other hand, the non-aqueous organic solvent serves as a medium for transporting ions taking part in the electrochemical reaction of a lithium secondary battery.

The non-aqueous organic solvent may include a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent.

The carbonate-based solvent may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and/or the like. The ester-based solvent may include methyl acetate, ethyl acetate, n-propyl acetate, dimethylacetate, methylpropionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, and/or the like.

The ether-based solvent may include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and/or the like, and the ketone-based solvent may include cyclohexanone, and/or the like.

The alcohol-based solvent may include ethanol, isopropyl alcohol, and/or the like, and the aprotic solvent may include nitriles such as T-CN (wherein T is a hydrocarbon group having a C₂ to C₂₀ linear, branched, or cyclic structure and may include a double bond, an aromatic ring, or an ether bond), and/or the like, dioxolanes such as 1,3-dioxolane, and/or the like, sulfolanes, and/or the like.

The non-aqueous organic solvent may be used alone or in a mixture. When the organic solvent is used in a mixture, the mixture ratio may be controlled in accordance with a desirable battery performance.

The carbonate-based solvent is prepared by mixing a cyclic carbonate and a linear carbonate. When the cyclic carbonate and linear carbonate are mixed together in a volume ratio of 1:1 to 1:9, an electrolyte performance may be improved.

The non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent in addition to the carbonate-based solvent. Herein, the carbonate-based solvent and the aromatic hydrocarbon-based organic solvent may be mixed in a volume ratio of 1:1 to 30:1.

The aromatic hydrocarbon-based organic solvent may be an aromatic hydrocarbon-based compound of Chemical Formula 2.

In Chemical Formula 2, R₁ to R₆ are the same or different and are each independently selected from the group consisting of hydrogen, halogen, C₁ to C₁₀ alkyl group, haloalkyl group, and a combination thereof.

Specific examples of the aromatic hydrocarbon-based organic solvent may be selected from benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and a combination thereof.

The electrolyte of a lithium secondary battery may further include vinyl ethylene carbonate, vinylene carbonate, an ethylene carbonate-based compound represented by Chemical Formula 3, or a combination thereof, in order to improve cycle life of a battery.

In Chemical Formula 3, R₇ and R₈ are each independently selected from hydrogen, a halogen, a cyano group (CN), a nitro group (NO₂), and a fluorinated C₁ to C₅ alkyl group, provided that at least one of R₇ and R₈ is selected from a halogen, a cyano group (CN), a nitro group (NO₂), and a fluorinated C₁ to C₅ alkyl group, and R₇ and R₈ are not simultaneously hydrogen.

Examples of the ethylene carbonate-based compound may be difluoro ethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, fluoroethylene carbonate, and the like. The amount of the additive for improving cycle life may be used within an appropriate range.

The lithium salt dissolved in the non-aqueous solvent supplies lithium ions in a battery, enables a basic operation of a lithium secondary battery, and improves transportation of the lithium ions between positive and negative electrodes. Examples of the lithium salt include at least one supporting salt selected from LiPF₆, LiSbF₆, LiAsF₆, LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄, LiPO₂F₂, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (wherein x and y are natural numbers, for example, an integer from 0 to 20, for example, 1 to 20), lithium difluoro(bisoxolato) phosphate) (LiDFOP), LiCl, LiI, LiB(C₂O₄)₂ (lithium bis(oxalato) borate; LiBOB), and lithium difluoro(oxalate) borate (LiDFOB). The lithium salt may be used in a concentration from 0.1 M to 2.0 M. When the lithium salt is included at the above concentration range, an electrolyte may have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity.

FIG. 1 is a schematic view showing a structure of a lithium secondary battery according to an embodiment of this disclosure.

Referring to FIG. 1, a lithium secondary battery 100 according to an embodiment may include an electrode assembly 40 and a case 50 in which the electrode assembly 40 is contained.

The electrode assembly 40 includes a positive electrode 10 including a positive active material, a negative electrode 20 including the negative active material, and a separator 30 disposed between the positive electrode 10 and the negative electrode 20. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated in the aforementioned electrolyte solution (not shown) according to the present disclosure.

The positive electrode 10 includes a current collector and a positive active material layer disposed on the current collector and including a positive active material.

In the positive active material layer, the positive active material may include a compound (lithiated intercalation compound) being capable of intercalating and deintercallating lithium and specifically at least one composite oxide of lithium and a metal of cobalt, manganese, nickel, and a combination thereof may be used. Specific examples thereof may be a compound represented by one of the following chemical formulae. Li_(a)A_(1-b)X_(b)D₂ (0.90≤a≤1.8, 0≤b≤0.5); Li_(a)A_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li_(a)E_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li_(a)E_(2-b)X_(b)O_(4-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li_(a)Ni_(1-b-c)Co_(b)X_(c)D_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2); Li_(a)Ni_(1-b-c)Co_(b)X_(c)O_(2-α)T_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)Ni_(1-b-c)Co_(b)X_(c)O_(2-α)T₂ (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)D_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α≤2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-α)T_(α) (0.90≤a·1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-α)T₂ (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂ (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1); Li_(a)Ni_(b)Co_(c)Mn_(d)G_(e)O₂ (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li_(a)NiG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)CoG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1-b)G_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1-g)G_(g)PO₄ (0.90≤a≤1.8, 0≤g≤0.5); QO₂; QS₂; LiQS₂; V₂O₅; LiV₂O₅; LiZO₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃ (0≤f≤2); Li_((3-f))Fe₂(PO₄)₃ (0≤f≤2); and Li_(a)FePO₄ (0.90≤a≤1.8).

In the chemical formulae, A is selected from Ni, Co, Mn, and a combination thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, and a combination thereof; D is selected from O, F, S, P, and a combination thereof; E is selected from Co, Mn, and a combination thereof; T is selected from F, S, P, and a combination thereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and a combination thereof; Q is selected from Ti, Mo, Mn, and a combination thereof; Z is selected from Cr, V, Fe, Sc, Y, and a combination thereof; and J is selected from V, Cr, Mn, Co, Ni, Cu, and a combination thereof.

Particularly, in one embodiment, the positive active material may preferably be a nickel-containing lithium transition metal compound (e.g., a nickel-included lithium transition metal compound) positive active material. For example, the positive active material may be a high-Ni positive active material having nickel in a content of 60 mol % or more in the nickel-containing lithium transition metal compound. That is, nickel may be included at 60 mol % (molar percent) or more based on the total molar amount of all the transition metals included in the lithium transition metal compound. The high-Ni positive active material may be deteriorated (e.g., may cause the deterioration) due to Ni²⁺ self-reduction and a side reaction with residual lithium (reaction of moisture and CO₂) on a surface of the positive electrode. However, the use of the negative electrode with Si at an amount of about 0.1 wt % to about 32 wt % based on 100 wt % of the total weight of the negative active material and the electrolyte with the additive including the compound of Chemical Formula 1 may effectively prevent or substantially prevent these shortcomings. In contrast, the effects of using the negative electrode and the electrolyte according to one or more embodiments may not be significant when a cobalt-based active material is used as the positive active material. In particular, it is not desirable to use the electrolyte according to an embodiment in a high-voltage battery using the cobalt-based active material to partially oxidize a phosphorous-based film which may be generated on a surface of the positive electrode to decompose it. That is, the use of the electrolyte according to one or more embodiments in a high-voltage battery using the cobalt-based active material is not desirable, due to the decomposition of a phosphorous-based film, which may be generated on a surface of the positive electrode, through partial oxidization.

According to an embodiment, the nickel-containing lithium transition metal compound positive active material may be one represented by Chemical Formula 4. Li_(p)(Ni_(x)Co_(y)Me_(z))O₂[  Chemical Formula 4]

In Chemical Formula 4, 0.9≤p≤1.1, 0.6≤x≤0.98, 0<y≤0.3, 0<z≤0.3, x+y+z 1, and Me is at least one of Al, Mn, Mg, Ti, and Zr.

More specifically, in Chemical Formula 4, x may be in the range of 0.7≤x≤0.98.

The lithium metal oxide may have a coating layer on the surface, or may be mixed with another lithium metal oxide having a coating layer. The coating layer may include at least one coating element compound selected from an oxide of a coating element, a hydroxide of a coating element, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, and a hydroxy carbonate of a coating element. The compound for the coating layer may be amorphous or crystalline. The coating element included in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer may be disposed by a method having no adverse influence on properties of a positive active material by using these elements in the compound, (e.g., the method may include any coating method (e.g., spray coating, dipping, etc.)), but is not illustrated in more detail since it is well-known to those skilled in the related field.

In the positive electrode, the positive active material may be included in an amount of 90 wt % to 98 wt % based on a total weight of the positive active material layer.

In an embodiment, the positive active material layer may include a binder and a conductive material. Herein, the binder and the conductive material may be included in an amount of 1 wt % to 5 wt %, respectively based on the total amount of the positive active material layer.

The binder serves to adhere the positive active material particles with one another and to adhere the positive active material to a current collector and examples thereof may be polyvinyl alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an epoxy resin, nylon, and the like, but are not limited thereto.

The conductive material is included to provide conductivity to the electrode and any electrically conductive material may be used as a conductive material unless it causes a chemical change. Examples of the conductive material may include a carbon-based material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, and/or the like; a metal-based material of a metal powder or a metal fiber including copper, nickel, aluminum, silver, and/or the like; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

The current collector may be an aluminum foil, a nickel foil, or a combination thereof, but is not limited thereto.

The negative electrode 20 includes a current collector and a negative active material layer disposed on the current collector and including a negative active material.

The negative active material may include a material that reversibly intercalates/deintercalates lithium ions, a lithium metal, a lithium metal alloy, a material capable of doping/dedoping lithium, or a transition metal oxide.

The material that reversibly intercalates/deintercalates lithium ions may include for example, a carbon material, and i.e., the carbon material which may be a generally-used carbon-based negative active material in a lithium secondary battery. Examples of the carbon-based negative active material may include crystalline carbon, amorphous carbon, or mixtures thereof. The crystalline carbon may be unspecified shaped, or sheet, flake, spherical, or fiber shaped natural graphite or artificial graphite. The amorphous carbon may be a soft carbon, a hard carbon, a mesophase pitch carbonization product, fired coke, and/or the like.

The lithium metal alloy includes an alloy of lithium and at least one additional metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

The material being capable of doping/dedoping lithium may be Si, SiO_(x) (0<x<2), a Si-Q alloy (wherein Q is an element selected from an alkali metal, an alkaline-earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof, and not Si), a Si-carbon composite, Sn, SnO₂, Sn—R (wherein R is an element selected from an alkali metal, an alkaline-earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof, and not Sn), a Sn-carbon composite, and/or the like. At least one of these materials may be mixed with SiO₂. The elements Q and R may each independently be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof.

The transition metal oxide may include lithium titanium oxide.

Particularly, in one embodiment, the negative active material may desirably include Si at an amount of about 0.1 wt % to about 32 wt % based on 100 wt % of the total weight of the negative active material. As such, when the negative active material including Si at an amount of the above range, higher capacity may be achieved (e.g., exhibited).

The negative active material including Si at an amount of about 0.1 wt % to about 32 wt % may be i) a material including a core including a mixture of crystalline carbon and Si, and an amorphous carbon coating layer surrounding the core, or ii) a material including a core including crystalline carbon and Si adhered to a surface of the core. That is, in one embodiment, the negative active material (including Si at an amount of about 0.1 wt % to about 32 wt %) may include a core and an amorphous carbon coating layer surrounding the core, wherein the core includes a mixture of crystalline carbon and Si. In another embodiment, the negative active material (including Si at an amount of about 0.1 wt % to about 32 wt %) may include a core and Si adhered to a surface of the core, wherein the core includes crystalline carbon.

When the negative active material includes i) a material including a core including a mixture of crystalline carbon and Si, and an amorphous carbon coating layer surrounding the core, the amount of crystalline carbon may be about 38 wt % to about 94.9 wt % based on 100 wt % of the total weight of the negative active material, and the amount of the amorphous carbon may be about 5 wt % to about 30 wt %. Furthermore, the amorphous carbon coating layer may be substantially totally surrounded on the surface of the core, that is, may be a layer which may be continuously presented on the surface of the core. For example, the amorphous carbon coating layer may be a continuously layer that covers substantially the whole surface of the core. In another embodiment, the coating layer may be presented to partially expose the surface of the core, that is, may be an island-type which may be discontinuously presented on the surface of the core. For example, the coating layer may have an island shape and forms a discontinuous layer on the surface of the core, which does not cover the whole surface of the core and exposes a portion of the surface of the core. When the negative active material is ii) a material including a core including crystalline carbon and Si adhered to the surface of the core, the amount of the crystalline carbon may be about 99.9 wt % to about 68 wt %. In addition, Si may be presented on the surface of the core in the form of the island-type which is discontinuously presented to partially expose the surface of the core. For example, Si may be presented on the surface of the core in the shape of islands and forms a discontinuous layer, which exposes a portion of the surface of the core.

The negative active material layer includes a negative active material and a binder, and optionally a conductive material.

In the negative active material layer, the negative active material may be included in an amount of 95 wt % to 99 wt % based on the total weight of the negative active material layer. In the negative active material layer, a content of the binder may be 1 wt % to 5 wt % based on a total weight of the negative active material layer. When the negative active material layer includes a conductive material, the negative active material layer includes 90 wt % to 98 wt % of the negative active material, 1 wt % to 5 wt % of the binder, and 1 wt % to 5 wt % of the conductive material.

The binder improves binding properties of negative active material particles with one another and with a current collector. The binder may use a non-water-soluble binder, a water-soluble binder, or a combination thereof.

The non-water-soluble binder may be polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

The water-soluble binder may be a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, a copolymer of propylene and a C₂ to C₈ olefin, a copolymer of (meth)acrylic acid and (meth)acrylic acid alkyl ester, or a combination thereof.

When the water-soluble binder is used as a negative electrode binder, a cellulose-based compound may be further used to provide viscosity as a thickener. The cellulose-based compound includes one or more of carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metals may be Na, K, or Li. The thickener may be included in an amount of 0.1 parts by weight to 3 parts by weight based on 100 parts by weight of the negative active material.

The conductive material is included to provide electrode conductivity and any electrically conductive material may be used as a conductive material unless it causes a chemical change. Examples of the conductive material include a carbon-based material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, denka black, a carbon fiber, and the like; a metal-based material of a metal powder or a metal fiber including copper, nickel, aluminum silver, and the like; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

The current collector may include one selected from a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, and a combination thereof.

The positive active material layer and the negative active material layer are formed by mixing an active material, a binder and optionally a conductive material in a solvent to prepare an active material composition, and coating the active material composition on a current collector. The formation method of the active material layer is well known, and thus is not described in detail in the present disclosure. The solvent includes N-methylpyrrolidone and the like, but is not limited thereto. When a water-soluble binder is used in the negative active material layer, a solvent used for preparing the negative active material composition may be water.

The separator 30 may include polyethylene, polypropylene, polyvinylidene fluoride, and multi-layers thereof such as a polyethylene/polypropylene double-layered separator, a polyethylene/polypropylene/polyethylene triple-layered separator, or a polypropylene/polyethylene/polypropylene triple-layered separator.

Hereinafter, the disclosure (e.g., the subject matters of the disclosure) will be specifically examined (e.g., illustrated) through Examples.

Example 1

(1) Positive Electrode and Negative Electrode

97.3 wt % of LiCoO₂ as a positive active material, 1.4 wt % of polyvinylidene fluoride as a binder, and 1.3 wt % of ketjen black as a conductive material were mixed and then, dispersed in N-methylpyrrolidone to prepare positive active material slurry. The positive active material slurry was coated on an aluminum foil and then, dried and compressed to manufacture a positive electrode.

98 wt % of graphite as a negative active material, 1 wt % of polyvinylidene fluoride as a binder, and 1 wt % of ketjen black as a conductive material were mixed and then, dispersed in N-methylpyrrolidone to prepare a negative active material layer composition, and the negative active material layer composition was coated on a copper foil and dried to manufacture a negative electrode.

(2) Preparation of Electrolyte

0.95 M LiPF₆ was added to a mixed solution of ethylene carbonate (EC):diethyl carbonate (DEC):ethylpropionate (EP) mixed in a volume ratio of 30:50:20 to prepare a non-aqueous mixed solution.

1.0 wt % of a compound represented by Chemical Formula 1-1 based on an amount of the non-aqueous mixed solution was added thereto to manufacture an electrolyte for a lithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

The positive and negative electrodes manufactured according to the (1) and the electrolyte prepared according to the (2) were used to manufacture a lithium secondary battery cell.

Example 2

(1) Manufacture of Positive Electrode and Negative Electrode

A positive electrode and a negative electrode were manufactured in the same method as Example 1.

(2) Manufacture of Electrolyte

0.95 M LiPF₆ was added to a first mixed solution of ethylene carbonate (EC):ethylmethyl carbonate (EMC):ethylpropionate (EP):γ-butyrolactone (GBL) used in a volume ratio of 27:50:20:3 to prepare a second mixed solution.

6 wt % of fluoroethylene carbonate (FEC), 0.5 wt % of vinyl ethylene carbonate (VEC), 0.2 wt % of lithium tetrafluoroborate (LiBF₄), 5 wt % of succinonitrile (SN), 2 wt % of hexane tri-cyanide (HTCN), and 0.5 wt % of a compound represented by Chemical Formula 1-1 based on an amount of the second mixed solution were added thereto to prepare an electrolyte for a lithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

A prismatic lithium secondary battery was manufactured by housing the positive and negative electrodes and a polypropylene separator in a container and injecting the prepared electrolyte thereinto.

Example 3

A lithium secondary battery cell was manufactured according to the same method as Example 2 except that 0.25 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Example 4

A lithium secondary battery cell was manufactured according to the same method as Example 2 except that 1 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Example 5

A lithium secondary battery cell was manufactured according to the same method as Example 2 except that 2 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Comparative Example 1

(1) Manufacture of Positive Electrode and Negative Electrode

A positive electrode and a negative electrode were manufactured according to the same method as Example 1.

(2) Manufacture of Electrolyte

0.95 M LiPF₆ was added to a mixed solution of ethylene carbonate (EC):diethyl carbonate (DEC):ethylpropionate (EP) used in a volume ratio of 30:50:20 to prepare a non-aqueous mixed solution.

(3) Manufacture of Lithium Secondary Battery Cell

A prismatic lithium secondary battery cell was manufactured by housing the positive and negative electrodes manufactured in the above (1) and the electrolyte prepared in the above (2).

Comparative Example 2

(1) Manufacture of Positive Electrode and Negative Electrode

A positive electrode and a negative electrode were manufactured according to the same method as Example 1.

(2) Manufacture of Electrolyte

An electrolyte for a lithium secondary battery cell was manufactured according to the same method as Example 2 except that the compound represented by Chemical Formula 1-1 was not added thereto.

(3) Manufacture of Lithium Secondary Battery Cell

A lithium secondary battery cell was manufactured according to the same method as Example 2.

Experimental Example 1—CV Characteristics Evaluation

Cyclic voltammetry (CV) characteristics of the half-cells according to Example 1 and Comparative Example 1 were evaluated. The result of Example 1 is shown in FIG. 2, and the result of Comparative Example 1 is shown in FIG. 3.

In FIGS. 2 and 3, 1, 3, and 5 denote the number of cycles.

Referring to FIGS. 2 and 3, currents were largely increased during both of intercalation/deintercalation of lithium in FIG. 2 compared with FIG. 3. Accordingly, when the lithium secondary battery cell including the electrolyte using additives including the compound represented by Chemical Formula 1-1 according to Example 1 was compared with the lithium secondary battery cell according to Comparative Example 1, lithium ions were relatively more easily intercalated/deintercalated.

Experimental Example 2—Evaluation of LSV Characteristics

The electrolytes of Example 1 and Comparative Example 1 were evaluated regarding oxidation electrode decomposition at 25° C. using a linear sweep voltammetry (LSV) method. In this evaluation, a three-electrode electrochemical cell using a Pt electrode as a working electrode, a counter electrode, and Li as a reference electrode was used. Herein, a scan was performed in a range of 3.0 V to 7.0 V at a rate of 1 mV/sec, and the results are shown in FIGS. 4A and 4B. FIG. 4A shows LSV evaluation results of Example 1 and Comparative Example 1, and FIG. 4B is an enlarged view showing a y-axis scale of FIG. 4A.

Referring to FIGS. 4A and 4B, the electrolyte including additives including the compound represented by Chemical Formula 1-1 according to Example 1 exhibited an onset potential increase at a higher voltage compared with the electrolyte according to Comparative Example 1. Accordingly, when the additive of Chemical Formula 1-1 was added, oxidation resistance of an electrolyte was increased.

Experimental Example 3—Thickness Increase Rate

The lithium secondary battery cells according to Example 2 and Comparative Example 2 were constant current charged at a current density of 1 C until a voltage reached 4.45 V. After measuring thicknesses of the cells after the charge, their thickness variation ratios (%) were measured every seven day, while stored at 60° C. for 28 days. The thickness variation ratios measured at the 28th day were shown in Table 1.

TABLE 1 Thickness Division increase rate (%) Example 2 14.3 Example 3 22.9 Example 4 9.0 Example 5 11 Comparative 23.8 Example 2

Referring to Table 1 and FIG. 5, the lithium secondary battery cells each using the electrolyte including the additive of the present invention according to each of the example embodiments exhibited a significantly reduced thickness compared with the lithium secondary battery cells each not including the additive according to Comparative Example.

Experimental Example 4—Cycle Characteristics

The lithium secondary battery cells according to Example 2 and Comparative Example 2 were constant current charged at current density of 1 C at 45° C. until a voltage reached 4.45 V. Subsequently, the lithium secondary battery cells were allowed to stand for 10 minutes and then, discharged at constant current of 1 C, until the voltage reached 3 V, and this charge and discharge as one cycle was 250 times repeated. The results are shown in FIG. 5.

Referring to FIG. 5, the lithium secondary battery cells according to Examples exhibited excellent high temperature cycle-life characteristics compared with the lithium secondary battery cell according to Comparative Example.

Example 6

(1) Positive Electrode and Negative Electrode

97.3 wt % of LiNi_(0.88)Co_(0.1)Al_(0.02)O₂ as a positive active material, 1.4 wt % of polyvinylidene fluoride as a binder, and 1.3 wt % of ketjen black as a conductive material were mixed and then dispersed in N-methylpyrrolidone to prepare a positive active material slurry. The positive active material slurry was coated on an aluminum foil and then dried and compressed to manufacture a positive electrode.

98 wt % of a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 99 wt % and Si at 1 wt %), 1 wt % of polyvinylidene fluoride as a binder, and 1 wt % of ketjen black as a conductive material were mixed and then dispersed in N-methylpyrrolidone to prepare a negative active material layer composition, and the negative active material layer composition was coated on a copper foil and dried to manufacture a negative electrode.

(2) Preparation of Electrolyte

1.5 M LiPF₆ was added to a mixed solution of ethylene carbonate (EC):ethyl methyl carbonate (EMC):dimethyl carbonate (DMC) mixed in a volume ratio of 20:10:70 to prepare a non-aqueous mixed solution.

3.0 wt % of fluoroethylene carbonate and 0.5 wt % of a compound represented by Chemical Formula 1-1 based on 100 wt % of the non-aqueous mixed solution were added thereto to manufacture an electrolyte for a lithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

The positive and negative electrodes manufactured according to (1), the electrolyte prepared according to (2), and a polypropylene coated with Al₂O₃ separator (CCS) were used to manufacture a lithium secondary battery cell.

Example 7

A lithium secondary battery cell was manufactured according to the same method as Example 6, except that 1 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Example 8

A lithium secondary battery cell was manufactured according to the same method as Example 6, except that 2 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Example 9

A lithium secondary battery cell was manufactured according to the same method as Example 7, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 95 wt % and Si at 5 wt %) was used as a negative active material.

Example 10

A lithium secondary battery cell was manufactured according to the same method as Example 7, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 88 wt % and Si at 12 wt %) was used as a negative active material.

Example 11

A lithium secondary battery cell was manufactured according to the same method as Example 6, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 68 wt % and Si at 32 wt %) was used as a negative active material.

Example 12

A lithium secondary battery cell was manufactured according to the same method as Example 7, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 68 wt % and Si at 32 wt %) was used as a negative active material.

Example 13

A lithium secondary battery cell was manufactured according to the same method as Example 8, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 68 wt % and Si at 32 wt %) was used as a negative active material.

Example 14

A lithium secondary battery cell was manufactured according to the same method as Example 6, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 99.9 wt % and Si at 0.1 wt %) was used as a negative active material.

Example 15

A lithium secondary battery cell was manufactured according to the same method as Example 6, except that 0.1 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Reference Example 1

(1) Positive Electrode and Negative Electrode

A positive electrode was manufactured according to the same method as Example 6.

98 wt % of graphite as a negative active material, 1 wt % of polyvinylidene fluoride as a binder, and 1 wt % of ketjen black as a conductive material were mixed and then dispersed in N-methylpyrrolidone to prepare a negative active material slurry. The negative active material slurry was coated on a copper foil, and then dried and compressed to manufacture a negative electrode.

(2) Preparation of Electrolyte

1.15 M LiPF₆ was added to a mixed solution of ethylene carbonate (EC):ethyl methyl carbonate (EMC):dimethyl carbonate (DMC) mixed in a volume ratio of 20:10:60 to prepare a non-aqueous mixed solution. 3 wt % of fluoroethylene carbonate and 0.05 wt % of a compound represented by Chemical Formula 1-1 based on 100 wt % of the non-aqueous mixed solution were added thereto to manufacture an electrolyte for a lithium secondary battery.

(3) Manufacture of Lithium Secondary Battery Cell

The positive and negative electrodes manufactured according to (1), the electrolyte prepared according to (2), and a polypropylene coated with Al₂O₃ separator (CCS) were used to manufacture a lithium secondary battery cell.

Reference Example 2

A lithium secondary battery cell was manufactured according to the same method as Example 7, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 50 wt % and Si at 50 wt %) was used as a negative active material.

Reference Example 3

A lithium secondary battery cell was manufactured according to the same method as Example 11, except that 5 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Reference Example 4

A lithium secondary battery cell was manufactured according to the same method as Example 15, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 99.95 wt % and Si at 0.05 wt %) was used as a negative active material.

Reference Example 5

A lithium secondary battery cell was manufactured according to the same method as Example 15, except that a negative active material including a graphite core and Si adhered to the surface of the core (graphite core at 67 wt % and Si at 33 wt %) was used as a negative active material.

Reference Example 6

A lithium secondary battery cell was manufactured according to the same method as Reference Example 5, except that 0.05 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Reference Example 7

A lithium secondary battery cell was manufactured according to the same method as Reference Example 6, except that 4 wt % of the compound represented by Chemical Formula 1-1 was added to prepare an electrolyte.

Experimental Example 5—Amount of the Generated Gas

The lithium secondary battery cells according to Examples 6 to 15 and Reference Examples 1 to 7 were each stored at 60° C. for 30 days and the amount of the generated gas was measured. The results are shown in Table 2.

Experimental Example 6—DC-IR (Direct Current Internal Resistance)

The lithium secondary battery cells according to Examples 6 to 15 and Reference Examples 1 to 7 were each stored at 60° C. for 30 days and the direct-current internal resistance was measure by measuring a voltage drop (V) which occurred while a current was flowed at 1 C for 1 second under a SOC50 condition (charged to be 50% of charge capacity based on 100% of the entire battery charge capacity, i.e., fully charged). The results are shown in Table 2.

TABLE 2 Increase Total amount rate of of generated resistance Amount of Si Amount of a gas after after in negative compound of allowing to allowing to active Chemical stand at a stand at a material Formula 1-1 high temper- high temper- (wt %) (wt %) ature (g/cc) ature (%) Example 6 1 0.5 0.066 143 Example 7 1 1 0.061 135 Example 8 1 2 0.052 129 Example 9 5 1 0.063 142 Example 10 12 1 0.075 151 Example 11 32 0.5 0.120 169 Example 12 32 1 0.110 161 Example 13 32 2 0.101 149 Example 14 0.1 0.5 0.067 145 Example 15 1 0.1 0.090 145 Reference 0 0.05 0.130 167 Example 1 Reference 50 1 0.172 180 Example 2 Reference 32 5 0.190 159 Example 3 Reference 0.05 0.1 0.101 150 Example 4 Reference 33 0.1 0.131 180 Example 5 Reference 33 0.05 0.130 167 Example 6 Reference 33 4 0.120 169 Example 7

As shown in Table 2, the lithium battery cells each including a negative active material including Si at an amount of about 1 wt % to about 32 wt % and an electrolyte including an additive compound of Chemical Formula 1-1 according to Examples 6 to 15 each exhibited a surprisingly low amount of generated gas and a low resistance increase at high-temperature storage, compared to the lithium battery cells including Si at a large amount of 50 wt % (e.g., Reference Example 2), even though the electrolyte includes the same additive compound.

Furthermore, Reference Examples 3 and 7, each including a large amount of the additive of 5 wt % or 4 wt %, respectively, Reference Examples 1 and 6, each including a small amount of the additive of 0.05 wt %, and Reference Examples 4 and 5, each including Si at an amount of 0.05 wt % or 33 wt %, respectively, exhibited abruptly increased amount of generated gas and resistance increase at high-temperature storage.

As used herein, the term “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those of ordinary skill in the art.

Also, any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” is intended to include all subranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. Accordingly, Applicant reserves the right to amend this specification, including the claims, to expressly recite any sub-range subsumed within the ranges expressly recited herein.

While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, and on the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.

DESCRIPTION OF SYMBOLS

-   -   100: secondary battery     -   10: positive electrode     -   20: negative electrode     -   30: separator     -   50: case 

What is claimed is:
 1. A lithium secondary battery, comprising: a positive electrode comprising a positive active material; a negative electrode comprising a negative active material; and an electrolyte comprising a non-aqueous organic solvent, a lithium salt, and an additive comprising a compound represented by Chemical Formula 1, wherein the negative active material comprises Si at about 0.1 wt % to about 32 wt % in amount based on a total amount of the negative active material:

and wherein, in Chemical Formula 1, A is a substituted or unsubstituted aliphatic chain or (—C₂H₄—O—C₂H₄-)n, and n is an integer from 1 to
 10. 2. The lithium secondary battery of claim 1, wherein in Chemical Formula 1, A is a C₂ to C₂₀ hydrocarbon chain or (—C₂H₄—O—C₂H₄-)n, and n is an integer from 1 to
 5. 3. The lithium secondary battery of claim 1, wherein the compound represented by Chemical Formula 1 is a compound represented by Chemical Formula 1-1:


4. The lithium secondary battery of claim 1, wherein the additive is 0.1 wt % to 3 wt % in amount based on a total amount of the electrolyte.
 5. The lithium secondary battery of claim 1, wherein the additive is 0.15 wt % to 2 wt % in amount based on a total amount of the electrolyte.
 6. The lithium secondary battery of claim 1, wherein the negative active material comprises: i) a core comprising a mixture of crystalline carbon and Si, and an amorphous carbon coating layer surrounding the core; or ii) a core comprising crystalline carbon and Si adhered to a surface of the core.
 7. The lithium secondary battery of claim 1, wherein the additive further comprises an additional additive selected from fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, succinonitrile, hexane tricyanide, lithium tetrafluoroborate, and propane sultone.
 8. The lithium secondary battery of claim 7, wherein the additional additive is 0.1 wt % to 20 wt % in amount based on a total amount of the electrolyte.
 9. The lithium secondary battery of claim 1, wherein the positive active material comprises a nickel-containing lithium transition metal compound.
 10. The lithium secondary battery of claim 9, wherein nickel is about 60 mol % or more in amount based on 100 mol % of all transition metals in the nickel-containing lithium transition metal compound.
 11. The lithium secondary battery of claim 9, wherein the positive active material comprises a lithium metal oxide represented by Chemical Formula 4: Li_(p)(Ni_(x)CO_(y)Me_(z))O₂  [Chemical Formula 4] wherein, in Chemical Formula 4, 0.9≤p≤1.1, 0.6≤x≤0.98, 0<y≤0.3, 0<z≤0.3, x+y+z=1, and Me is at least one selected from Al, Mn, Mg, Ti, and Zr.
 12. The lithium secondary battery of claim 6, wherein the negative active material comprises the core comprising a mixture of crystalline carbon and Si, and the amorphous carbon coating layer surrounding the core, and wherein the amorphous carbon coating layer covers substantially the whole surface of the core.
 13. The lithium secondary battery of claim 6, wherein the negative active material comprises the core comprising crystalline carbon and Si adhered to the surface of the core, and wherein silicon is in a shape of islands and forms a discontinuous layer on the surface of the core.
 14. The lithium secondary battery of claim 1, wherein the lithium salt is selected from LiPF₆, LiSbF₆, LiAsF₆, LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄, LiCl, LiI, LiB(C₂O₄)₂, LiDFOP, LiDFOB, LiPO₂F₂, and LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂), and wherein x and y are each independently an integer from 0 to
 20. 